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| Earth’s magnetic field protects us from the solar wind by deflecting the charged particles. And for some reason, the field has been drifting westward. Credit: CLAUS LUNAU/Getty |
Over the 400 years or so
that humans have been measuring Earth's magnetic field, it has drifted
inexorably to the west. Now, a new hypothesis suggests that weird waves in
Earth's outer core may cause this drift.
The slow waves, called
Rossby waves, arise in rotating fluids. They're also known as "planetary
waves," and they're found in many large, rotating bodies, including on
Earth in the oceans and atmosphere and on Jupiter and the sun.
Earth's outer core is also a
rotating fluid, meaning Rossby waves circulate in the core, too. Whereas
oceanic and atmospheric Rossby waves have crests that move westward against
Earth's eastward rotation, Rossby waves in the core are "a bit like
turning atmospheric Rossby waves inside out," said O.P. Bardsley, a
doctoral student at the University of Cambridge in England, and the author of a
new study on the Rossby wave hypothesis. Their crests always move east.
The rotation of magnetic
iron in Earth's core gives rise to the planet's geomagnetic field. The
geomagnetic field, in turn, protects the planet from solar radiation, making it
important for life on Earth. Without it, the planet's surface would be
bombarded by charged particles streaming from the sun that would ultimately rip
away Earth's atmosphere.
While trying to understand
the waves that propagate throughout Earth's core, Bardsley realized that some
of these waves might explain one of the mysteries of the planet's magnetic
field. Over the past four centuries, scientists have made measurements of magnetic
declination — the difference between true north and the point where a compass
needle points. (Because the magnetic field is chock-full of little local
anomalies, the compass needle moves around a little compared to true north
depending on where you're standing.)
Throughout those four
centuries, the anomalies revealed by these declination measurements have shown
a tendency to move westward, Bardsley reported in the new research, which was
published today (May 15) in the journal Proceedings of the Royal Society A.
"The westward drift
manifests itself primarily as a series of blobs over the Atlantic near the
equator," Bardsley told Live Science, and they drift at around 10.5 miles
(17 kilometers) per year.
Theories to explain the
drift have typically focused on the dynamics of the outer core. The most
popular hypothesis, Bardsley said, is that the outer core contains a gyre
similar to the atmosphere's jet stream, which happens to be moving westward and
is dragging Earth's magnetic field along with it. The problem, Bardsley said,
is that there's no particular reason why this gyre should exist. It might very
well exist, he said, but given that there is no direct evidence, other
explanations are still possible.
One possibility, Bardsley said,
is that Rossby waves explain the weirdness of the magnetic field on Earth's
surface. This is a little odd, Bardsley said, because Rossby waves in the core
have eastward-moving crests, quite opposite the westward-moving drift. But
crests of waves don't always represent their total energy movement.
"It is entirely
possible to have a group of waves where the crests themselves are going east
but the [bulk of the] energy is going westward," Bardsley said. Something similar can even
happen with water waves. Their crests typically travel in the same direction of
the bulk of their energy, Bardsley said, but not necessarily at the same speed.
Surface measurements of the
geomagnetic field capture the bulk of energy movement, Bardsley said, but not
all the wiggly little details. So Rossby waves with a large-scale tendency to
move energy westward could explain the westward drift measured over the
Atlantic Ocean. The small-scale details, like those eastward-moving crests,
would be impossible to detect.
The westward drift and the
Rossby wave hypothesis are largely unrelated to a more famous question
regarding the magnetic field: Is it going to flip? Periodically throughout
Earth's history, magnetic north and magnetic south have swapped places. This
isn't particularly problematic, except that it takes about 10,000 years,
Bardsley said, and the process causes an uptick in anomalies and a weakening of
the magnetic field in between the poles.
A weakened field can let
more solar particles through, which can disrupt electric grids and cause
problems with navigational systems. However, scientists aren't certain whether
the weakening of the magnetic field over the past century or two is a sign of
an impending flip-flop or merely a recoverable wobble.
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